Method for forming narrow trench structures

ABSTRACT

A method for forming narrow trench structures. A substrate covered by a layer to be defined is provided. A plurality of oxidable first masking islands is formed on the layer to be defined. Thereafter, the first masking islands are oxidized to form an oxide layer on the sidewall and the upper surface of each first masking island. A second masking island is formed in each gap between the oxidized first masking islands. The oxide layers are subsequently removed to form narrow openings between the first and second masking islands, having a width substantially equal to the thickness of the removed oxide layer. The layer to be defined underlying the narrow openings is etched to form the narrow trench structures on the substrate. Finally, the first and second masking islands are removed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a semiconductor process, andmore particularly, to a method for forming narrow trench structures.

2. Description of the Related Art

In order to increase the integration of integrated circuits (ICs), onetrend in the semiconductor industry is to make the semiconductor devicesor device-to-device spacing as small as possible, thereby enablingfabrication of more semiconductor devices in the predetermined area on achip to raise the operating speed and performance of ICs.

Metal oxide semiconductor (MOS) transistors are common semiconductordevices. The fabrication of the MOS transistor includes successivelyforming a gate dielectric layer, a conductive layer, and a photoresistlayer on a substrate. Thereafter, lithography is performed on thephotoresist layer to form gate patterns therein. Finally, the gatepatterns are transferred onto the conductive layer by etching.Accordingly, line/space width is limited by the resolution of theavailable lithography equipment. As a result, the lithography resolutionis critical for increasing device density. In other words, the ICintegration is limited by photoresist properties and the lightwavelength for exposure. The lithography resolution can be raised byspecific photoresist or lithography equipment, but the fabrication costmay increase. In light of the foregoing, there exists a need for amethod to increase the integration of ICs without being limited bylithography.

U.S. Pat. No. 5,254,218 discloses a method for forming narrow isolatedtrenches, wherein a conformable polysilicon layer is formed on thesurface of the masking islands formed on a substrate and covers thesubstrate. Thereafter, a silicon oxide masking layer is formed on bothsides of each masking island and covers the polysilicon layer on thesubstrate. Finally, the polysilicon layer on the upper surface andsidewall of each masking island is removed to form openings for definingnarrow trenches. In this method, no specific photoresist or lithographyequipment is required. The uniformity of the polysilicon layer formed byCVD with poor step coverage, is however reduced, due to the varyingdimensions of the narrow trenches. Moreover, since the polysilicon layeron the substrate is covered by the silicon oxide masking layer, undercutoccurs during etching of the uncovered polysilicon layer. As a result,the dimensions of the narrow trenches are difficult to preciselycontrol.

Additionally, U.S. Pat. No. 6,355,528 discloses a method to form narrowstructures using a dual damascene process, which employs an anisotropicetching to form spacers on the sidewalls of the masking islands. Thespace between each masking island is subsequently filled with anothermasking layer. Finally, the spacers are removed to form narrow openingsfor forming the narrow structures. The spacers formed by anisotropicetching, however, cannot have a vertical profile. As a result, thedimensions of the narrow structures are also difficult to preciselycontrol.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for forming narrow trench structures and a method for forminggate structures with narrow spacings, which employ the space left byremoving spacers to narrow the intervals between devices, therebyincreasing the device density without being limited by lithography.

It is another object of the invention to provide a method for formingnarrow trench structures and a method for forming gate structures withnarrow spacings, which uses oxidation to form oxide layers serving asspacers, thereby increasing the uniformity of the spacer thickness toaccomplish an easily controlled and stable process.

According to the object of the invention, a method for forming gatestructures with narrow spacings is provided. First, a substrate isprovided. Next, a dielectric layer, a polysilicon layer, and a cappinglayer are successively formed on the substrate. A plurality of siliconislands is subsequently formed on the capping layer. The silicon islandsare oxidized to form an oxide layer on the sidewall and the uppersurface of each silicon island. Next, a masking layer is formed in eachgap between the oxidized silicon islands. Next, the oxide layers areremoved to form a narrow opening between each of the silicon islands andthe masking layers, having a width substantially equal to the thicknessof the removed oxide layer. Thereafter, the cap layer and thepolysilicon layer underlying the narrow openings are etched to form thegate structures with narrow spacings on the substrate. Finally, thesilicon islands, the masking layers, and the capping layer are removed.

Moreover, the capping layer can be a silicon nitride layer and themasking layer can be a photoresist or silicon layer.

Additionally, according to the object of the invention, a method forforming narrow trench structures is provided. First, a substrate coveredby a layer to be defined is provided and a plurality of oxidable firstmasking islands is subsequently formed on the layer to be defined.Thereafter, the first masking islands are oxidized to form an oxidelayer on the sidewall and the upper surface of each first maskingisland. Next, a second masking island is formed in each gap between theoxidized first masking islands. The oxide layers are subsequentlyremoved to form narrow openings between the first and second maskingislands, having a width substantially equal to the thickness of theremoved oxide layer. Next, the layer to be defined underlying the narrowopenings is etched to form the narrow trench structures on thesubstrate. Finally, the first and second masking islands are removed.

Additionally, the layer to be defined can be a silicon layer, a metallayer, or a dielectric layer.

Moreover, the first masking island can be composed of silicon and thesecond masking island can be composed of photoresist or silicon.

DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,given by way of illustration only and thus not intended to be limitativeof the present invention.

FIGS. 1 a to 1 f are cross-sections showing a method for forming narrowtrench structures according to the invention.

FIGS. 2 a to 2 f are cross-sections showing a method for forming gatestructures with narrow spacings according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a, a substrate 100, such as a silicon substrate or othersemiconductor substrate, is provided. The substrate 100 may contain avariety of elements, including, for example, transistors, capacitors,and other semiconductor elements as are well known in the art. Thesubstrate 100 may also contain other insulating layers or metalinterconnect layers. Here, a flat substrate is depicted for simplicity.

Next, a layer to be defined 102 is deposited on the substrate 100 byconventional deposition. In the invention, the layer to be defined 102may be a silicon layer, a metal layer, or other well known dielectriclayer used in the semiconductor process. Thereafter, a capping layer 104with a thickness of about 50 to 1000 Å is optionally formed on the layerto be defined 102 to serve as a protective layer. In the invention, thecapping layer 104 is composed of an anti-oxidation material, such as asilicon nitride layer.

Next, a masking layer 106 is formed on the capping layer 104 to serve asan etch mask. In the invention, the masking layer 106 may be composed ofan oxidable material. Moreover, the oxidized film has a high etchingselectivity with respect to the masking layer 106, such as silicon.Next, a photoresist layer 108 with island patterns is formed on themasking layer 106 by lithography, as shown in FIG. 1 a.

Next, in FIG. 1 b, the masking layer 106 uncovered by the photoresistlayer 108 is removed by, for example, reactive ion etching (RIE), toform a plurality of masking islands 106 a. Thereafter, the photoresistlayer 108 is removed by ashing or other suitable solutions. A criticalstep of the invention is subsequently performed. That is, spacers areformed on the sidewalls of masking islands 106 a. For example, maskingislands 106 a are thermally oxidized to form an oxide layer 106 b on thesidewall and the upper surface of each masking island 106 a, wherein theoxide layer 106 b formed on the sidewall of each masking island 106 a isused as the spacer. As mentioned above, since the capping layer 104 iscomposed of an anti-oxidation material, oxygen diffusing into theunderlying layer to be defined 102 can be prevented. Here, the width ofthe narrow trench structure is based on the thickness of the oxide layer106 b. Accordingly, the temperature and the length of the thermaloxidation can be determined according to the demand.

Next, in FIG. 1 c, another masking layer (not shown) is formed on theoxide layer 106 b and fills each gap 109 between the masking islands 106a. In the invention, the masking layer may have a high etchingselectivity with the oxide layer 106 b or a higher polishing rate thanthe oxide layer 106 b. For example, the masking layer is composed ofphotoresist or silicon, with photoresist being preferable. Thereafter,the masking layer over the masking islands 106 a is removed by etchingback or conventional polishing, such as chemical mechanical polishing(CMP), using the oxide layers 106 b as stoppers to leave a portion ofmasking layer 110 in each gap 109.

Next, in FIG. 1 d, the oxide layers 106 b are removed by conventionaldry or wet etching to simultaneously expose the surfaces of the maskingislands 106 a and the capping layer 104 and form a narrow opening 112between each of the masking islands 106 a and the remaining maskinglayers 110, having a width substantially equal to the thickness of theremoved oxide layer 106 b.

Next, in FIG. 1 e, the capping layer 104 and the layer to be defined 102underlying the openings 112 are successively etched using the maskingislands 106 a and the remaining masking layers 110 as etch masks to formopenings 114 and expose the substrate 100. Finally, the masking islands106 a, the remaining masking layers 110, and the capping layer 104 areremoved to complete the fabrication of the narrow trench structures 116,as shown in FIG. 1 f.

FIGS. 2 a to 2 f are cross-sections showing a method for forming gatestructures with narrow spacings according to the invention. First, inFIG. 2 a, a substrate 200, such as a silicon substrate or othersemiconductor substrate, is provided. The substrate 200 may contain avariety of elements, including, for example, transistors, capacitors,and other semiconductor elements as are well known in the art. Thesubstrate 100 may also contain other insulating layers or metalinterconnect layers. Here, a flat substrate is depicted for simplicity.

Next, a dielectric layer 202 and a polysilicon layer 204 are depositedon the substrate 200. Here, the dielectric layer 202 may be a siliconoxide layer formed by thermal oxidation to serve as a gate dielectriclayer. Moreover, the polysilicon layer 204 may be formed by conventionaldeposition, such as chemical vapor deposition (CVD), for defining thegate electrode. Thereafter, a capping layer 206 with a thickness ofabout 50 to 500 Å is formed on the polysilicon layer 204. The cappinglayer 206 may be composed of an anti-oxidation material, such as asilicon nitride layer.

Next, a masking layer 208 is formed on the capping layer 206 to serve asan etch mask for defining the gate electrode. In the invention, themasking layer 208 may be composed of an oxidable material. Moreover, theoxidized film has a high etching selectivity with respect to the maskinglayer 208, such as silicon.

Next, in FIG. 2 b, a plurality of masking islands 208 a is formed on thecapping layer 206 by lithography and etching. A critical step of theinvention is subsequently performed. That is, spacers are formed on thesidewalls of masking islands 208 a. For example, masking islands 208 aare thermally oxidized to form an oxide layer 208 b on the sidewall andthe upper surface of each masking island 208 a, wherein the oxide layer208 b formed on the sidewall of each masking island 208 a is used as thespacer. As mentioned above, since the capping layer 206 is composed ofan anti-oxidation material, oxygen diffusing into the underlyingpolysilicon layer 204 can be prevented. Here, the width of the narrowspacings between the gate structures is based on the thickness of theoxide layer 208 b. Accordingly, the temperature and the length of thethermal oxidation can be determined according to the demand.

Next, in FIG. 2 c, another masking layer (not shown) is formed on theoxide layer 208 b and fills in each gap 209 between the masking islands208 a. In the invention, the masking layer may a have high etchingselectivity with the oxide layer 208 b or a higher polishing rate thanthe oxide layer 208 b. For example, the masking layer is composed ofphotoresist or silicon, with photoresist being preferable. Thereafter,the masking layer over the masking islands 208 a is removed by etchingback or conventional polishing, such as CMP, using the oxide layers 208b as stoppers to leave a portion of masking layer 210 in each gap 209.

Next, in FIG. 2 d, the oxide layers 208 b are removed by conventionaldry or wet etching to simultaneously expose the surfaces of the maskingislands 208 a and the capping layer 206 and form a narrow opening 212between each of the masking islands 208 a and the remaining maskinglayers 210, having a width substantially equal to the thickness of theremoved oxide layer 208 b.

Next, in FIG. 2 e, the capping layer 206, the polysilicon layer 204, andthe dielectric layer 202 underlying the openings 212 are successivelyetched using the masking islands 208 a and the remaining masking layers210 as etch masks to form openings 214 and expose the substrate 200.Finally, the masking islands 208 a, the remaining masking layers 210,and the capping layer 206 are removed to complete the fabrication of thegate structures 116 with narrow spacings, as shown in FIG. 2 f.

According to the invention, the dimensions of the narrow trench or thedimensions of the gap between each gate structure is based on thethickness of the oxide spacer formed by thermal oxidation. Accordingly,device-to-device spacing can be reduced without being limited bylithography, thereby increasing device density. That is, integration ofthe integrated circuits can be increased. Additionally, compared to therelated art, the spacer of the invention is formed by thermal oxidation,which offers better uniformity of thickness and vertical profile.Accordingly, the dimensions of the narrow trench or the dimensions ofthe gap between each gate structure can be precisely and stablycontrolled.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation toencompass all such modifications and similar arrangements.

1. A method for forming gate structures with narrow spacings, comprisingthe steps of: providing a substrate; successively forming a dielectriclayer, a polysilicon layer, and a capping layer on the substrate;forming a plurality of silicon islands on the capping layer; oxidizingthe silicon islands to form an oxide layer on the sidewall and the uppersurface of each silicon island; forming a masking layer in each gapbetween the oxidized silicon islands; removing the oxide layers to forma narrow opening between each of the silicon islands and the maskinglayers, having a width substantially equal to the thickness of theremoved oxide layer; successively etching the cap layer and thepolysilicon layer underlying the narrow openings to form the gatestructures with narrow spacings on the substrate; and removing thesilicon islands, the masking layers, and the capping layer.
 2. Themethod as claimed in claim 1, wherein the capping layer is a siliconnitride layer.
 3. The method as claimed in claim 1, wherein the maskinglayer is a photoresist layer.
 4. The method as claimed in claim 1,wherein the masking layer is a silicon layer.
 5. A method for formingnarrow trench structures, comprising the steps of: providing a substratecovered by a layer to be defined; forming a plurality of oxidable firstmasking islands on the layer to be defined; oxidizing the first maskingislands to form an oxide layer on the sidewall and the upper surface ofeach first masking island; forming a second masking island in each gapbetween the oxidized first masking islands; removing the oxide layers toform narrow openings between the first and second masking islands, eachone having a width substantially equal to the thickness of the removedoxide layer; etching the layer to be defined underlying the narrowopenings to form the narrow trench structures on the substrate; andremoving the first and second masking islands.
 6. The method as claimedin claim 5, further forming a capping layer on the layer to be definedbefore forming the first masking islands.
 7. The method as claimed inclaim 6, wherein the capping layer is a silicon nitride layer.
 8. Themethod as claimed in claim 6, wherein the layer to be defined is asilicon layer.
 9. The method as claimed in claim 6, wherein the layer tobe defined is a metal layer.
 10. The method as claimed in claim 5,wherein the layer to be defined is a dielectric layer.
 11. The method asclaimed in claim 5, wherein the first masking island is silicon.
 12. Themethod as claimed in claim 5, wherein the second masking island isphotoresist.
 13. The method as claimed in claim 5, wherein the secondmasking island is silicon.